Even though verse 19 prophesies the destruction of the devil’s kingdom, this verse and verses 21–23 which follow are clearly describing events before the destruction of the devil’s kingdom. Thus, “at that day” should be read as meaning “continuing until that day.”
This verse describes one way in which Satan influences human beings—by taking over their hearts so that they prefer evil to good. Being stirred up to anger against good symbolizes rejecting the good and embracing evil. When such people encounter good, they feel angry. They feel enough of the light of Christ that they remember the attractiveness of good; but because they have accepted evil, they resist the impulse toward repentance (which would certainly include pain along with the ultimate promise of cleanliness) by responding instead with anger directed toward the good. Such anger justifies their rejection of the good and spares them the painful process of repentance.